1. Field of the Invention
The present invention relates to measuring connectivity between different pore types in porous media of subsurface formations, and more particularly to measuring connectivity with low-field nuclear magnetic resonance and fast field cycling nuclear magnetic resonance measurements.
2. Description of the Related Art
Nuclear magnetic resonance (or NMR) measurements whether in well logging of in situ formations or in laboratory testing of formation rock samples is used as a reliable porosity measurement technique since NMR measurements are not dependent on lithology. Thus, NMR data is often used for reserve estimation by identifying the movable and immovable fluids in the reservoir rocks. Accurate reserve estimation from NMR logging and laboratory data, however, is quite challenging due to the presence of diffusion coupling for complicated pore systems in carbonate rocks.
Currently, so far as is known, pore connectivity cannot be measured accurately from NMR logging data. Therefore, what is known as a NMR T2 cutoff value for NMR logging data is obtained from the NMR data, based on the laboratory measurement from a selected rock sample from the reservoir of interest. This method is an approximation and has been subject to a large margin of error due to its being based on statistical estimates. Other pore connectivity laboratory measurements are based on sonic and electric current resistivity within porous media, which is a different measure from fluid connectivity, since sonic and electric current resistivity do not take into account capillary pressure. Fluid connectivity is based on capillary pressure within the rock sample being a main factor which controls the fluid flow in the porous media.
Pore connectivity determination by X-ray microcomputed tomography (or microCT) techniques has been used. However, there are disadvantages. The X-ray microCT technique determines the rock structural connectivity based on an X-ray microcomputed tomographic image, which is different from pore connectivity of fluids in the porous media of the rock. It requires the cut-off value to determine the pores and solid matrix. X-ray microCT imaging requires a very small rock sample size to achieve a high resolution (˜4 μm/voxel) image. This size sample is the minimum resolution requirement to observe micro-pores of most carbonate rocks. This sample size is too small to be a representative volume of carbonate rocks which have relatively heterogeneous pore distribution. The most common sample size for a rock sample is a 1 mm3 cube, which is considerably larger than that required for X-ray microCT imaging.
Prior art techniques have been able to distinguish between fluid types (hydrocarbon and water) in rock samples pores by identifying biphasic behavior of water and hydrocarbon, but so far as is known not to determine connectivity between different pore types.